Heretofore, there have been three-group zoom lenses for every type of camera. Three-group zoom lenses are compact and are widely used. Examples of such lenses are disclosed in Japanese Laid-Open Patent Publication 3-240011, Japanese Laid-Open Patent Publication 59-31922 and in U.S. Pat. No. 4,647,160. However, the rapid dissemination of digital cameras and video cameras in recent years, coupled with the increased demand for small lenses having a high picture quality and low distortion, as in general cameras, has necessitated the satisfaction of unique conditions when employing fixed photographic image elements, such as CCD arrays.
In contrast to film, CCD arrays as used in digital cameras and video cameras can receive light efficiently only when the incident luminous flux is nearly perpendicular to the photographic image plane. For example, in the case of zoom lenses with a two-group construction of a positive group and a negative group, as used frequently in 35 mm compact cameras, the incidence angle onto the photographic image plane increases as the image height reaches the periphery of the image plane. This is especially true if the distance from the exit pupil to the photographic image plane is short. When using such an optical system to image optical flux onto a CCD array, the peripheral luminance will be greatly reduced as compared to that at the center of the image, and the so-called "shading" effect occurs. Accordingly, as a condition for satisfactorily using a two-group zoom lens for photographic imaging onto a CCD array, it becomes necessary to move the exit pupil to a position sufficiently remote from the photographic image plane.
However, with digital cameras and video cameras, autofocus is commonly a feature that is desired, and high speed focusing is often favored. For this reason, what is known as the inner focusing method and the rear focusing method are frequently used as focusing methods for zoom lenses in order to allow the zoom lens to be light-weight and for easily allowing the zoom lens focus position to be driven close to the camera body side.